I had first added a folder containing an aspect that I want to add to my
application using the Properties => AspectJ Build => Aspect Path tab.
Later I decided, I rather need to add that folder as a project source
folder. Doing so, I got an error, that the buildpath contains a duplicate
entry.
So I removed the entry from the Aspect Path tab (which is now empty), but I
still get that error message!

Meanwhile I restarted eclipse a few times (and the Aspect Path tab remained
empty), but still, when I try to add the aspect folder to my build path, I
keep getting that "duplicate" error msg. Where is that info kept, that a
folder was once added to the aspect path and why is that info not properly
cleaned up, when removing a folder from the Aspect path?